A door operator is known from the U.S. Pat. No. 5,018,304 and offers a variable torque clutch as a method of controlling operating torque and free egress. In many applications, especially linear accelerator doors, it is desirable, and some times mandated, that the door be movable by an external force below a predetermined magnitude. This is done, so that in an emergency situation such as during a power failure, it is still possible to open and close the door. In the patent to Longoria, an electromagnetic clutch is used which is supposed to slip at a predetermined value. This design has been found to be less than optimal with respect to when the clutch should and should not slip in actual field conditions. Research into presently available electromagnetic clutches, finds that present manufacturers do not recommend using electromagnetic clutches in slip clutch applications, and the electromagnetic clutches are preferably only to be used as an on/off clutch. In particular the potentiometer voltage control of these clutches is only for adjusting the start-up and running modes, and not meant to control slippage.
Automatic door operators are known for standard commercial doors, such as those seen in department stores. These swinging doors are mostly only powered in one direction and have a spring return for the other direction. Spring returns are known to have difficulty operating the movement of a door if particular problems such as out of plum or misalignment of the frame, or a pressure differential exists in a particular installation. The standard commercial door operators, because of their design, are usually limited to applications where the door weighs less than 4,500 pounds. When the weight of the door exceeds this amount, the standard controlling and driving devices experience problems.
One of the problems, such as in applications similar to U.S. Pat. No. 5,453,736 to Noren, is that the position of the door is measured using an encoder with a counter. Movement of the door generates a series of pulses, which are then counted to determine how much the door has moved. When power is interrupted, when electrical noise is present, and/or an automated door operator needs to be reset, encoder/counter systems need to move the door to a start or home position and then reset the counting. If the door operator of Noren was used on a linear accelerator door during a power failure, during electrical noise or reset, a door weighing up to 20,000 pounds is being moved without any microprocessor control, until the door is returned to its start or home position. This can be a dangerous situation.
Also the motor and drive train used for relatively light weight doors, are not significantly effected by the weight of the door. Braking or slowing of the door is therefore not a significant consideration. With doors weighing up to tens of thousands of pounds, the driving of the extra weight of the door is accomplished by using a much more powerful motor and drive train, however, such factors such as slowing, braking and the door overrunning the motor now become significant problems, as is the importance of power operation in both the forward and reverse directions.